The present invention relates to a gas turbine combustor for burning both liquid and gaseous fuel in compressed air. More specifically, the present invention relates to a low NOx combustor having the capability of burning lean mixtures of both liquid and gaseous fuel.
In a gas turbine, fuel is burned in compressed air, produced by a compressor, in one or more combustors. Traditionally, such combustors had a primary combustion zone in which an approximately stoichiometric mixture of fuel and air was formed and burned in a diffusion type combustion process. Fuel was introduced into the primary combustion zone by means of a centrally disposed fuel nozzle. When operating on liquid fuel, such nozzles were capable of spraying fuel into the combustion air so that the fuel was atomized before it entered the primary combustion zone. Additional air was introduced into the combustor downstream of the primary combustion zone so that the overall fuel/air ratio was considerably less than stoichiometric--i.e., lean. Nevertheless, despite the use of lean fuel/air ratios, the fuel/air mixture was readily ignited at start-up and good flame stability was achieved over a wide range of firing temperatures due to the locally richer nature of the fuel/air mixture in the primary combustion zone.
Unfortunately, use of rich fuel/air mixtures in the primary combustion zone resulted in very high temperatures. Such high temperatures promoted the formation of oxides of nitrogen ("NOx"), considered an atmospheric pollutant. It is known that combustion at lean fuel/air ratios reduces NOx formation. However, achieving such lean mixtures requires that the fuel be widely distributed and very well mixed into the combustion air. This can be accomplished by pre-mixing the fuel into the combustion air prior to its introduction into the combustion zone.
In the case of gaseous fuel, this pre-mixing can be accomplished by introducing the fuel into primary and secondary annular passages that pre-mix the fuel and air and then direct the pre-mixed fuel into primary and secondary combustion zones, respectively. The gaseous fuel is introduced into these primary and secondary pre-mixing passages using fuel spray tubes distributed around the circumference of each passage. A combustor of this type is disclosed in "Industrial RB211 Dry Low Emission Combustion" by J. Willis et al., American Society of Mechanical Engineers (May 1993).
Unfortunately, such combustors are capable of operation on only gaseous fuel because the fuel spray tubes are not adapted to atomize liquid fuel into the combustor. Liquid fuel spray nozzles, such as those used in conventional rich-burning combustors, are known. However, sufficient mixing of the fuel and air to achieve adequately lean fuel/air ratios can not be achieved by merely incorporating such a nozzle into the pre-mixing passage. This is so because such liquid fuel spray nozzles do not completely atomize the fuel, resulting in the formation of large fuel droplets and locally rich fuel/air mixtures.
It is therefore desirable to provide a gas turbine combustor having primary and secondary passages for pre-mixing gaseous fuel in combustion air that is also capable of pre-mixing a liquid fuel in at least one of the gas pre-mixing passages.